Group III nitride compound semiconductor such as gallium nitride (GaN) (hereinafter also referred to as a “Group III nitride semiconductor” or a “GaN-based semiconductor”) has been gaining attention as a material for semiconductor elements that emit blue or ultraviolet light. A blue laser diode (LD) is used for high-density optical disk devices or displays while a blue light emitting diode (LED) is used for displays, lighting, etc. It is expected to use an ultraviolet LD in the field of biotechnology or the like and an ultraviolet LED as an ultraviolet source for a fluorescent lamp.
Generally, Group III nitride semiconductor substrates (for example, GaN substrate) that are used for LDs or LEDs are formed by vapor phase epitaxy. For instance, Group III nitride crystals are grown heteroepitaxially on a sapphire substrate. However, the quality of crystals obtained through vapor phase epitaxy has a problem. More specifically, crystals obtained by this method generally have a dislocation density of 108 cm−2 to 109 cm−2 and thus reducing dislocation density has been an important issue. In order to deal with this issue, efforts have been made to reduce the dislocation density and thereby, for example, an epitaxial lateral overgrowth (ELOG) method has been developed. With this method, the dislocation density can be reduced, but the implementation thereof is complicated, which poses a problem in practical utilization.
On the other hand, besides the vapor phase epitaxy, a method of carrying out crystal growth from the liquid phase also has been studied. At the beginning, the liquid phase growth method required super high pressures and super high temperatures. In this connection, a method has been developed in which crystals are grown in Na flux, thus enabling the alleviation of the pressure and temperature conditions up to about 50 atm (50×1.01325×105 Pa) at about 700° C. Recently, single crystals whose maximum crystal size is about 1.2 mm can be obtained by a method in which a mixture of Ga and Na is melted in a nitrogen gas atmosphere containing ammonia at 800° C. and 50 atm (50×1.01325×105 Pa), and then crystals are grown for 96 hours using the melt (see, for instance, Patent document 1). Further, a crystal growth apparatus and a growth method have been proposed in which pressure is applied to and heat is applied externally to a reactor vessel (for example, see Patent document 2).
FIG. 17 shows one example of the manufacturing apparatus used for a liquid phase growth method (see Patent document 3). As shown in this drawing, in this apparatus, a reactor vessel 720 is stored in a pressure-resistant vessel 702. Inside the pressure-resistant vessel 702, there is a space surrounded by a heat insulator 711, on an inner wall of which a heater 710 is placed. The reactor vessel 720 is placed in such a space. A pressure regulator 770 is placed at the top of the pressure-resistant vessel 702. A lid 721 is attached to the reactor vessel 720, and a through hole 724 is formed in this lid 721.
GaN crystals are manufactured using this apparatus, for example, as follows. First, Ga and N are put in the reactor vessel 720, and this reactor vessel 720 is stored in the pressure-resistant vessel 702. Then, pressure is applied to the pressure-resistant vessel 702 in a gas atmosphere containing nitrogen while applying heat thereto by the heater 710, so that Ga and Na are melted in the reactor vessel 720. In this drawing, numeral 731 denotes the thus melted Ga and Na. Nitrogen is dissolved in the melted Ga and Na, thus leading to the generation of GaN and the growth of crystals.
Another manufacturing apparatus also has been proposed, including two pressure regulators, one of which supplies gas to a reactor vessel and the other supplies gas to a pressure-resistant vessel, thus allowing gas system is to be controlled independently between in the reactor vessel and in the pressure-resistant vessel provided outside of the reactor vessel (for example, see Patent document 4).    Patent document 1: JP 2002-293696 A    Patent document 2: JP 2001-102316 A    Patent document 3: JP 2002-68897 A    Patent document 4: JP 2001-58900 A